1. Field of the Invention
This invention relates to a swash plate type compressor used for automobile air conditioners, and more particularly it relates to a bearing device including a substantially hemispherical shoe interposed between the swash plate and piston of a swash plate type compressor so as to convert the rotary motion of the swash plate into the reciprocating motion of the piston.
Swash plate type compressors are of two types, one in which the angle of inclination of the swash plate with respect to the driving shaft is fixed, and the other, or the variable volume type, in which the angle of inclination of the swash plate with respect to the driving shaft is variable, to thereby vary the piston stroke. Further, in another aspect they are classified into two types, the single-acting type in which the delivery stroke is executed only when the piston is moved in one direction, and the double-acting type using a double-headed piston. As far as the piston-cylinder mechanism and the bearing section are concerned, these types are of the same basic construction.
2. Description of the Prior Art
A swash plate type compressor, as FIG. 4 shows its schematic arrangement, has a driving shaft 3 having a swash plate 2 obliquely attached thereto, the driving shaft 3 being rotatably supported in a cylinder block 5. The cylinder block 5 is formed with a plurality of cylinder bores 6 extending parallel with the driving shaft 3 and disposed circumferentially at equal intervals, each cylinder bore 6 having a piston 4 slidably fitted therein. The piston 4 has at one end a recess 4a formed therein to straddle the outer periphery of the swash plate 2, and spherical seats 4b are formed in the axially opposed surfaces of the recess 4a. A shoe 1 is incorporated in each spherical seat 4b such that it is interposed between the swash plate 2 and the piston 4. And the shoe 1 and the spherical seat 4b cooperate with each other to provide a bearing section A for converting the rotary motion of the swash plate into the reciprocating motion of the piston 4. That is, when the swash plate 2 is rotated with the driving shaft 3, the rotary motion of the swash plate 2 is converted into the reciprocating motion of the piston 4 by the action of the bearing section A. At this time, the shoe 1 performs a slide movement in one direction with respect to the swash plate 2 while with respect to the spherical seat 4b it performs a swing slide movement over a given angle.
There has heretofore been known an arrangement wherein, as shown in FIGS. 5A and 5B, of the convex spherical outer surface of the shoe 1, the contact portion and skirt portion which contact the spherical seat 4b are formed with different curvatures (see Japanese Patent Publication Heisei 3-51912). FIGS. 5A and 5B show the shoe 1 incorporated in the spherical seat 4b of the piston 4. The spherical seat 4b is formed using a single radius of curvature. On the other hand, the outer peripheral surface of the shoe 1 is composed of a reference spherical surface 1a at the top having substantially the same radius of curvature as the spherical seat 4b, and a skirt spherical surface 1b resulting from the skirt portion, which undergoes repetitive engagement and disengagement with and from the spherical seat 4b, receding from the reference spherical surface 1a toward the center of the shoe 1. In other words, not only does the curvature of the skirt spherical surface 1b differ from that of the reference spherical surface 1a but also the curvatures of other portions gradually vary. Thereby, the clearance which gently increases from the boundary between the reference spherical surface 1a and the skirt spherical surface 1b will vary in size as the shoe 1 swings, assisting in wedge action to effectively feed lubricating oil to the contact region of the reference spherical surface 1a which is sliding.
In the swash plate type compressor, the shoe is forced to perform the so-called precessional movement attending on the rotary movement of the swash plate, tending to suffer local abutment which, in turn, causes drawbacks including local wear. Therefore, to secure accurate abutment for the shoe, it is necessary to conduct management, in the process of producing such shoes, so as to ensure that the position of the abutment falls within a predetermined range. If, however, the outer peripheral surface of the shoe which contacts the spherical seat of the piston is a spherical surface having substantially the same radius of curvature as the spherical seat, the position of the abutment is not regular but tends to vary. Furthermore, the position of the abutment of the shoe against the spherical seat depends on the quality of finish of the spherical surface of the shoe, making it difficult to manage the height of the shoe, i. e., to manage the clearance between the shoe and the swash plate.
Accordingly, an object of this invention is to make it possible to provide a simple arrangement adapted to avoid contact between the top of a shoe and the spherical seat of a piston and secure a suitable contact area even if the angle of inclination of the swash plate is increased, thereby providing satisfactory lubrication.
A bearing device for swash plate type compressors according to the invention is characterized in that in a swash plate type compressor including a piston slidably received in a cylinder bore extending parallel with the driving shaft, the piston engaging the swash plate through a bearing device which swash plate is obliquely attached to the driving shaft, the rotation of the swash plate reciprocating the piston in the cylinder bore, the bearing device comprises a spherical seat composed of a single concave surface formed in the piston, and a substantially hemispherical shoe contacting the spherical seat, the outer peripheral surface of the shoe being composed of a skirt portion, a top portion having a larger radius of curvature than that of the spherical seat, and a transition portion positioned between the skirt portion and the top portion, having a smaller radius of curvature than that of the spherical seat, and contacting the latter. Making the radius of curvature of the top portion larger than that of the spherical seat ensures that even if there is a variation in the size of the outer peripheral surface of the shoe or the size of the spherical seat, there is no possibility of the top portion contacting the spherical seat; therefore, a suitable clearance is maintained between them to hold lubricating oil therein. Further, the transition portion between the skirt portion and the top portion is a portion which joins the skirt portion and the top portion, and the skirt portion and the top portion do not contact the spherical seat. The shoe contacts the spherical seat always at the transition portion.
The radius of curvature of the skirt portion of the shoe may advantageously be slightly smaller than that of the spherical seat. Making the radius of curvature of the skirt portion slightly smaller than that of the spherical seat forms a suitable clearance between the skirt portion and the spherical seat. Therefore, it is possible to prevent the edge of the corner of the spherical seat from abutting against the shoe, and to ensure satisfactory drawing of lubricating oil into the space between the shoe and the spherical seat.
The center of curvature of the skirt portion of the shoe may advantageously be located across the centerline of the shoe and radially spaced a predetermined distance from the centerline. In this case, since the outer peripheral surface of the shoe contacts the spherical seat always at the transition portion, the point at which the shoe abuts against the spherical seat can be accurately set. Further, a clearance can be formed between the skirt portion and the spherical seat by equalizing the radius of curvature of the skirt portion with the radius of curvature of the spherical seat or even by slightly decreasing it. it is preferable that the radius of curvature of the top portion of the shoe be set within the range of about 1.5-2.0 times the radius of curvature of the spherical seat. Making the radius of curvature of the top portion larger than that of the spherical seat makes it possible to avoid contact between the top portion of the shoe and the spherical seat of the piston, so that a suitable clearance is maintained between them to hold lubricating oil therein, as described above. If, however, the radius of curvature of the top portion exceeded twice the radius of curvature of the spherical seat, the smooth joint between the top portion and the transition portion would not be obtained and the smoothness with which the transition portion contacted the spherical seat would be impaired. Further, to exemplify the radius of curvature of the transition portion of the shoe in this case, it may be ⅓-⅔ times as large as the radius of curvature of the top portion.